1. Field of the Invention
The present invention relates to a method of inhibiting radioactive material deposition in the primary coolant structure of a nuclear power plant, and in particular relates to inhibiting the deposition of materials, especially materials containing .sup.60 Co and other radioactive cobalt materials on, for example, cooling water vessels, ducts, inner walls of water delivery equipment and inner surfaces of pipes in boiling water reactors and pressurized water reactors.
2. Description of the Related Art
In nuclear power stations using water-cooled reactors, radioactive materials contained in the primary cooling water may deposit on the inner wall surfaces of the pipes or of the primary coolant structure, and this leads to an increase of spatial dose rate in the primary cooling system.
When the primary coolant structure of the nuclear power station, for example the inner walls of cooling water vessels (pressure vessels), pipes, pumps and valves come in contact with cooling water at high temperature, corrosion products or dissolved substances are produced. These become radioactive due to bombardment with neutron radiation in the reactor, and are recycled in the primary cooling system together with the cooling water. During recirculation through the primary cooling system, the radioactive materials are deposited on the inner walls of pressure vessels, pipes or pumps, and the radiation dose rate in the system rises. The deposition of these radioactive materials, in particular .sup.60 Co which has a relatively long half-life (5.3 years), is undesirable in view of the need to reduce exposure of workers to radiation during shutdown of the nuclear power station, e.g. for regular inspection purposes.
Various methods may be envisaged to suppress increase of the radiation dose rate due to deposition of radioactive materials in the primary coolant structure of a water-cooled reactor. This may be done by inhibiting the production of radioactive materials (inhibiting corrosion of structures), localizing the radioactive materials where they are produced (the core), eliminating radioactive materials from cooling water (purification), inhibiting deposition of radioactive materials to structural surfaces, and elimination of the depositing radioactive materials (polishing, cleaning).
In U.S. Pat. No. 4,756,874 granted to Ruiz et al, a method is described for removing or inhibiting accumulation of radioactive cobalt on the surfaces or structural members by adding zinc ions to the cooling water of the water-cooled reactor.
However in this prior art patent, secondary radiation is produced due to radioactive conversion of the .sup.64 Zn isotope contained in the cooling water (the natural occurrence of .sup.64 Zn is 48 %, and the half-life of the .sup.65 Zn produced by radioactive conversion is 244 days). Therefore it was necessary to separate and remove .sup.64 Zn, and the high cost of this separation process was an obstacle to its practical application.
In addition to the above patent, another method is reported in Japanese Examined Patent Publication No. Hei 3-14155 where at least one metal ion chosen from the group Mg, Cr, V, Ti, Cu, Ni, Zn, Cd is added to the cooling water of the water-cooled reactor. However in the method described in this Japanese patent, there are difficulties from the viewpoint of maintaining the safety of the nuclear power station due to the following unresolved problems. That is, Cu accelerates corrosion in nickel-based alloys. Cd is a neutron absorbing material and toxic to man. Ti and V cannot exist as bivalent metal ions in cooling water and the mechanism by which they inhibit deposition of radioactive cobalt is unknown; moreover, the corrosive action they have on structural materials is unclear.
In view of the above situation, a new technique was required wherein the effect of secondary radiation could be ignored, and wherein there was no need for concern about adverse effect on structural materials.